The SFH618A-3 is a Vishay single-channel phototransistor optocoupler in a 4-DIP through-hole package. Its defining parametric is the current transfer ratio (CTR) bin: guaranteed minimum 100% and maximum 200% at a forward current of 1 mA. This 2:1 spread gives the designer a predictable drive range for the output transistor, which directly sets the load resistor value and the logic-level threshold on the receiving side. A CTR floor of 100% at only 1 mA input means the LED drive current can be kept low, saving power in isolated digital interfaces or PLC input modules.
Isolation voltage and temperature range
The 55 V output breakdown and 50 mA per-channel output current handle typical logic-level translation and low-power relay drive without extra buffering.
For faster interfaces like SPI or CAN isolation, a digital isolator with CMOS output stages would be the correct choice.
The through-hole 4-DIP package is straightforward to hand-solder or wave-solder, and the Tube shipping quantity suits both prototyping and medium-volume production runs.